Wet type paper friction material is manufactured such that fibrous base materials such as pulp and mixture materials such as friction modifiers are dispersed in water, and then formed into paper, and after its drying operation, the formed paper is impregnated with a thermosetting resin, thermoset, and molded under pressure. Since the function of the wet type paper friction material is influenced through blending of fibrous base materials, fillers and friction modifiers mainly acting as major constituents of such paper as above, improvement in selection about raw materials and contents of raw materials has been continued in compliance with application of the wet type paper friction material.
It has been known that the fibrous constituent has an important role to apply influence over porous characteristic and flexibility of the friction material and also apply a frictional characteristic and a strength and that the friction modifiers may also influence against the porous characteristic and a surface shape of it.
In the case that a so-called slip lock-up control is carried out at a slip lock-up mechanism for an automatic transmission for transferring a driving force while a rotational difference is being produced between the wet type paper friction material and its mating surface, it is required that a variation in coefficient of friction against a variation in sliding speed within a range of low speed (.mu.-v characteristic), a so-called "frictional characteristic" shows a positive relation over the entire range of speed to be controlled in slipping operation in order to prevent a frictional vibration called as a shudder from being produced. This .mu.-v characteristic is influenced in particular by flexibility of the frictional material or surface shape thereof. In general, as strength of the wet type paper frictional material is increased, its flexibility is deteriorated, an alignment with the mating surface is also deteriorated and then its frictional characteristic is deteriorated. In turn, in order to improve an alignment with the mating surface, application of flexibility to the wet type paper frictional material caused the frictional characteristic to be improved, although its fatigue strength under compression was decreased and both superior frictional characteristics and fatigue strength under compression could not be accomplished concurrently.